Charles Dharapak/AP Photo

The road to Mitt Romney's rally this morning in Shelby, Mich., - the state where his father was governor - was lined for more than a quarter mile not by his own signs, but by over a hundred for his rival Rick Santorum.

Romney's backers quickly snapped up the Santorum signs and replaced them with their own, but whether or not Romney himself will have such an easy time discarding Santorum in next week's Michigan primary is another matter. Suddenly, in a turn of events few could have predicted earlier in this primary season, Romney is embarking on a make-or-break week in his bid for the Republican presidential nomination.

If Romney wins Michigan and Arizona - the two states voting next Tuesday - the nomination will again be his to lose. He will have won six states in all, fought off stern challenges from Santorum in Michigan and Gingrich last month in Florida, and he will head into Super Tuesday with all the momentum - and the air of inevitability - at his back.

Lose Michigan to Santorum, though, and Romney will have lost the state of his birth, the state his father governed for six years. He will have lost more states in the primary than he has won  and he will trigger a slew of doubts among GOP leadership about whether the party needs to look elsewhere for its eventual nominee.

" If Romney cannot win Michigan, we need a new candidate," a veteran Republican senator told ABC News' Jon Karl last week.

Another indication of Romney's vulnerability: his financial advantage, once considered a strength, could now be seen as a sign of his failure to seal the deal with his party's voters.

While the former Massachusetts governor raked in $6.5 million in January, he plowed through around $19 million in a bid to win the first four voting states, and ended up with only a .500 record to show for it. In addition, Romney's Super PAC Restore Our Future spent nearly $14 million, mostly to air negative ads about Romney's rivals. But after all that, Romney has arguably lost the frontrunner mantle to Santorum, who has won three of the last five states and now leads Romney by 10 percentage points in the latest Gallup tracking poll.

While Romney can always dig into his own pockets to bankroll his campaign if necessary, his inability to put away Santorum and Gingrich early has opened the door for a protracted primary battle, since the two of them are each funded by single backers - Foster Friess for Santorum and Sheldon Adelson for Gingrich - who have been so encouraged by the first six weeks of voting that they have shown no sign of withdrawing their support.

The sooner Romney can start to slam that door shut, the sooner he will be able to focus his attacks not on Santorum and Gingrich but on President Obama.

"Re-election campaigns - in order for the challenging party to succeed - have to focus on the job performance of the incumbent," Republican strategist Carlos Curbelo noted in a recent interview. "As long as the spotlight is not on the president and his record, it's not good for the cause of defeating him."

To that end, Romney is shelling out big in a bid to win Michigan. He and his allies are spending more than twice as much as Santorum and his backers in Michigan. Romney's domination on the airwaves worked wonders in Florida and there are now signs it could be working well in Michigan, too.

According to the New York Times, Romney has risen in the polls in the past week to the point where the race in Michigan is now a toss-up, with Romney given a 51 percent chance of winning compared to 49 percent for Santorum.

"This is a huge, crucial moment. I think it's actually the most important moment for Romney in this entire campaign up until now," ABC News contributor Matthew Dowd said last week.

Romney will kick off the most pivotal week yet with a head-to-head battle with his rivals when the four remaining GOP candidates take the stage in Mesa, Ariz., Wednesday for a debate hosted by CNN. The last time Romney faced a crucial debate, he outperformed Santorum and Gingrich en route to winning the Florida primary.

Now more than ever, Romney could use a strong debate performance. Couple that with his ad buys in Michigan, his home state advantage, and his ability to do his best when his back is against the wall - and it would take a brave man to bet against Romney this week. Nothing less than the Republican presidential nomination might be at stake.

At a rally Monday in Cincinnati, Romney failed to energize a quiet, lackadaisical crowd, receiving only half-hearted applause during his speech.

Was it a sign, one that can't be concealed by his staff? In a week's time, we'll know for sure.

Matthew Jaffe is covering the 2012 campaign for ABC News and Univision.